Molecular fluorine laser with spectral linewidth of less than 1 pm

ABSTRACT

A narrow band molecular fluorine laser system includes an oscillator and an amplifier, wherein the oscillator produces a 157 nm beam having a linewidth less than 1 pm and the amplifier increases the power of the beam above a predetermined amount, such as more than one or several Watts. The oscillator includes a discharge chamber filled with a laser gas including molecular fluorine and a buffer gas, electrodes within the discharge chamber connected to a discharge circuit for energizing the molecular fluorine, and a resonator including the discharge chamber for generating a laser beam having a wavelength around 157 nm. Line-narrowing optics are included intra- and/or extra-resonator for reducing the linewidth of the laser beam to less than 1 pm. The amplifier may be the same or a different discharge chamber, and optical and/or electronic delays may be used for timing pulses from the oscillator to reach the amplifier at a maximum in the discharge current of the amplifier.

PRIORITY

[0001] This application claims is a rule 1.53(b) continuationapplication which claims the benefit of priority to U.S. patentapplication Ser. No. 09/599,130, filed Jun. 22, 2000, which claims thebenefit of priority to U.S. provisional patent applications No.60/140,531, filed Jun. 23, 1999, 60/204,095, filed May 15, 2000,60/162,735, filed Oct. 29, 1999, 60/166,967, filed Nov. 23, 1999 and60/170,342, filed Dec. 13, 1999, and which is also aContinuation-in-Part application claiming the benefit of priority toU.S. patent application Ser. No. 09/317,527, filed May 24, 1999, nowU.S. patent No. 6,154,470, which claims the benefit of priority to U.S.provisional patent applications No. 60/120,218, filed Feb. 12, 1999, and60/119,486, filed Feb. 10, 1999. All of the above priority applicationsare hereby incorporated by reference into the present application.

BACKGROUND OF INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a molecular fluorine lasersystem including line-narrowing elements and method for generating a VUVlaser beam having a spectral linewidth of less than substantially 1 pm.

[0004] 2. Description of the Related Art

[0005] Vacuum-UV microlithography takes advantage of the shortwavelength of the molecular fluorine laser (157.6 nm), which allows theformation of structures of 0.1 μm or below by photolithographic exposureon semiconductor substrates. TFT annealing and micro-machiningapplications may also be performed advantageously at this wavelength.

[0006] Given the limited choice of high quality optical materialsavailable in this wavelength range for manufacturing imaging lenses,requirements of minimal chromatic aberrations restrict spectrallinewidths of the laser source for refractive and partially achromaticimaging systems to below 1 pm. The expectation is that spectrallinewidths be between 0.1 pm and 0.2 pm, and perhaps even below 0.1 pmin the future. Conventional molecular fluorine lasers emit VUV beamshaving spectral linewidths of greater than 1 pm.

[0007] A disadvantage of narrowing of spectral linewidth in a laser isthat it commonly leads to a significant decrease of efficiency andoutput power. Therefore, it is recognized in the present invention thatto achieve a desired high throughput for 157 nm wafer steppers or waferscanners, it would be advantageous to have a line-narrowed molecularfluorine laser emitting an output beam of less than 1 pm, with a highoutput power that averages anywhere from several watts to more than 10watts.

SUMMARY OF THE INVENTION

[0008] It is therefore a first object of the present invention toprovide a VUV laser system having a narrow linewidth, i.e., less thansubstantially 1 pm for producing small structures on silicon wafers.

[0009] It is a second object of the invention to provide a VUV laserhaving a linewidth of 1 pm or less which exhibits sufficient outputpower, i.e., at least several Watts, to allow high throughput for VUVlithography applications at 157 nm.

[0010] Methods and apparatuses are provided in accord with the aboveobjects, such as a narrow band molecular fluorine laser system includingan oscillator and an amplifier, wherein the oscillator produces a 157 nmbeam having a linewidth less than 1 pm and the amplifier increases thepower of the beam above a predetermined amount, such as more than one orseveral Watts. The oscillator includes a discharge chamber filled with alaser gas including molecular fluorine and a buffer gas, electrodeswithin the discharge chamber connected to a discharge circuit forenergizing the molecular fluorine, and a resonator including thedischarge chamber and line-narrowing optics for generating the laserbeam having a wavelength around 157 nm and a linewidth less than 1 pm.

[0011] The amplifier preferably comprises a discharge chamber filledwith a laser gas including molecular fluorine and a buffer gas,electrodes connected to the same or a similar discharge circuit, e.g.,using an electrical delay circuit, for energizing the molecularfluorine. The amplifier discharge is timed to be at or near a maximum indischarge current when the pulse from the oscillator reaches theamplifier discharge chamber.

[0012] The line-narrowing optics preferably include one or more etalonstuned for maximum transmissivity of a selected portion of the spectraldistribution of the beam, and for relatively low transmissivity of outerportions of the spectral distribution of the beam. A prism beam expanderis preferably provided before the etalons for expanding the beamincident on the etalon or etalons. Two etalons may be used and tunedsuch that only a single interference order is selected.

[0013] The line-narrowing optics may further include a grating forselecting a single interference order of the etalon or etalonscorresponding to the selected portion of the spectral distribution ofthe beam. The resonator further preferably includes an aperture withinthe resonator, and particularly between the discharge chamber and thebeam expander. A second aperture may be provided on the other side ofthe discharge chamber.

[0014] The line-narrowing optics may include no etalon. For example, theline optics may instead include only a beam expander and a diffractiongrating. The beam expander preferably includes two, three or four VUVtransparent prisms before the grating. The grating preferably has ahighly reflective surface for serving as a resonator reflector inaddition to its role of dispersing the beam.

[0015] The line-narrowing optics may include an etalon output couplertuned for maximum reflectivity of a selected portion of the spectraldistribution of the beam, and for relatively low reflectivity of outerportions of the spectral distribution of the beam. This system wouldalso include optics such as a grating, dispersive prism or etalon,preferably following a beam expander, for selecting a singleinterference order of the etalon output coupler. The resonator wouldpreferably have one or more apertures for reducing stray light anddivergence within the resonator.

[0016] In any of above configurations including a grating, a highlyreflective mirror may be disposed after the grating such that thegrating and HR mirror form a Littman configuration. Alternatively, thegrating may serve to retroreflect as well as to disperse the beam in aLittrow configuration. A transmission grating or grism may also be used.

[0017] The buffer gas preferably includes neon and/or helium forpressurizing the gas mixture sufficiently to increase the output energyfor a given input energy and to increase the energy stability, gas andtube lifetime, and/or pulse duration. The laser system furtherpreferably includes a gas supply system for transferring molecularfluorine into discharge chamber and thereby replenishing the molecularfluorine, therein, and a processor cooperating with the gas supplysystem to control the molecular fluorine concentration within thedischarge chamber to maintain the molecular fluorine concentrationwithin a predetermined range of optimum performance of the laser.

[0018] The laser system may also include a spectral filter between theoscillator and the amplifier for further narrowing the linewidth of theoutput beam of the oscillator. The spectral filter may include an etalonor etalons following a beam expander. Alternatively, the spectral filtermay include a grating for dispersing and narrowing the beam. In thegrating embodiment, the spectral filter may include a lens focusing thebeam through a slit and onto a collimating optic prior to impinging uponthe beam expander-grating combination.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 schematically illustrates a molecular fluorine laser systemin accord with a preferred embodiment.

[0020]FIGS. 2a-2 f schematically show several alternative embodiments inaccord with a first aspect of the invention including various linenarrowing resonators and techniques utilizing line-narrowed oscillatorsfor the molecular fluorine laser.

[0021]FIG. 3a schematically shows a preferred embodiment in accord witha second aspect of the invention including an oscillator, a spectralfilter in various configurations, and an amplifier.

[0022]FIGS. 3b-3 d schematically show alternative embodiments ofspectral filters in further accord with the second aspect of theinvention.

[0023]FIG. 4a schematically shows an alternative embodiment in accordwith the second aspect of the invention including a single dischargechamber providing the gain medium for both an oscillator and anamplifier, and having a spectral filter in between.

[0024]FIG. 4b (i)-(iii) respectively show waveforms of the electricaldischarge current, un-narrowed beam intensity and output beam intensityin accord with the alternative embodiment of FIG. 3a.

[0025]FIG. 5a schematically shows a preferred embodiment in accord witha third aspect of the invention including a line-narrowed oscillatorfollowed by a power amplifier.

[0026]FIGS. 5b-5 f schematically show alternative embodiments ofline-narrowed oscillators in further accord with the third aspect of theinvention.

[0027]FIGS. 6a-6 b schematically show alternative embodiments in accordwith a fourth aspect of the invention including a single dischargechamber providing the gain medium for both an oscillator withline-narrowing and an amplifier.

Incorporation by Reference

[0028] What follows is a cite list of references each of which is, inaddition to those references cited above in the priority section, herebyincorporated by reference into the detailed description of the preferredembodiment below, as disclosing alternative embodiments of elements orfeatures of the preferred embodiments. A single one or a combination oftwo or more of these references may be consulted to obtain a variationof the preferred embodiments described in the detailed descriptionbelow. Further patent, patent application and non-patent references arecited in the written description and are also incorporated by referenceinto the preferred embodiment with the same effect as just describedwith respect to the following references:

[0029] 1- U. Stamm, “Status of 157 nm The 157 Excimer Laser”International SEMATECH 157 nm Workshop, Feb. 15-17 1999, Litchfield,Ariz., USA;

[0030] 2- T. Hoffman, J. M. Hueber, P. Das, S. Scholler, “Prospects ofHigh Repetition Rate F₂ (157 nm) Laser for Microlithography”,International SEMATECH 157 Workshop, Feb. 15-17 1999, Litchfield, Ariz.,USA;

[0031] 3- U. Stamm, I. Bragin, S. Govorkov, J. Kleinschmidt, R. Patzel,E. Slobodtchikov, K. Vogler, F. Voss, and D. Basting, “Excimer Laser for157 nm Lithography”, 24^(th) International Symposium onMicrolithography, Mar. 14-19,1999, Santa Clara, Calif., USA;

[0032] 4- T. Hoffman, J. M. Hueber, P. Das, S. Scholler, “Revisiting TheF₂ Laser For DUV microlithography”, 24^(th) International Symposium onMicrolithography, Mar. 14-19, 1999, Santa Clara, Calif., USA.

[0033] 5- W. Muckenheim, B. Ruckle, “Excimer Laser with Narrow Linewidthand Large Internal Beam Divergence”, J. Phys. E: Sci. Instrum. 20 (1987)1394;

[0034] 6- G. Grunefeld, H. Schluter, P. Andersen, E. W. Rothe,“Operation of KrF and ArF Tunable Excimer Lasers Without CassegrainOptics”, Applied Physics B 62 (1996) 241;

[0035] 7- U.S. patent applications Ser. Nos. 09/317,526, 09/343,333,60/122,145, 60/140,531, 60/162,735, 60/166,952, 60/171,172, 60/141,678,60/173,993, 60/166,967, 60/172,674, and 60/181,156, and U.S. patentapplication of Kleinschmidt, serial number not yet assigned, filed May18, 2000, for “Reduction of Laser Speckle in Photolithography byControlled Disruption of Spatial Coherence of Laser Beam,” and U.S. Pat.No. 6,005,880, each of which is assigned to the same assignee as thepresent application; and

[0036] 8- W. Mueckenheim, “Seven Ways to Combine Two Excimer Lasers,”reprinted from Jul. 1987 edition of Laser Focus/Electro-Optics.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] Referring to FIG. 1, a VUV laser system, preferably a molecularfluorine laser for deep ultraviolet (DUV) or vacuum ultraviolet (VUV)lithography, is schematically shown. Alternative configurations forlaser systems for use in such other industrial applications as TFTannealing and/or micromachining, e.g., are understood by one skilled inthe art as being similar to and/or modified from the system shown inFIG. 1 to meet the requirements of that application. For this purpose,alternative VUV laser system and component configurations are describedat U.S. patent applications Ser. Nos. 09/317,695, 09/317,526,09/317,527, 09/343,333, 60/122,145, 60/140,531, 60/162,735, 60/166,952,60/171,172, 60/141,678, 60/173,993, 60/166,967, 60/172,674, and60/181,156, and U.S. patent application of Kleinschmidt, serial numbernot yet assigned, filed May 18, 2000, for “Reduction of Laser Speckle inPhotolithography by Controlled Disruption of Spatial Coherence of LaserBeam,” and U.S. Pat. No. 6,005,880, each of which is assigned to thesame assignee as the present application and is hereby incorporated byreference.

[0038] The system shown in FIG. 1 generally includes a laser chamber 2having a pair or several pairs of main discharge electrodes 3 connectedwith a solid-state pulser module 4, and a gas handling module 6. Thesolid-state pulser module 4 is powered by a high voltage power supply 8.The laser chamber 2 is surrounded by optics module 10 and optics module12, forming a resonator. The optics modules 10 and 12 are controlled byan optics control module 14.

[0039] A computer 16 for laser control receives various inputs andcontrols various operating parameters of the system. A diagnostic module18 receives and measures various parameters of a split off portion ofthe main beam 20 via optics for deflecting a small portion of the beamtoward the module 18, such as preferably a beam splitter module 21, asshown. The beam 20 is preferably the laser output to an imaging system(not shown) and ultimately to a workpiece (also not shown). The lasercontrol computer 16 communicates through an interface 24 with astepper/scanner computer 26 and other control units 28.

[0040] The laser chamber 2 contains a laser gas mixture and includes apair of or several pairs of main discharge electrodes 3 and one or morepreionization electrodes (not shown). Preferred main electrodes 3 aredescribed at U.S. patent applications Ser. Nos. 09/453,670, 60/184,705and 60/128,227, each of which is assigned to the same assignee as thepresent application and is hereby incorporated by reference. Otherelectrode configurations are set forth at U.S. Pat. Nos. 5,729,565 and4,860,300, each of which is assigned to the same assignee, andalternative embodiments are set forth at U.S. Pat. Nos. 4,691,322,5,535,233 and 5,557,629, all of which are hereby incorporated byreference. The laser chamber 2 also includes a preionization arrangement(not shown). Preferred preionization units are set forth at U.S. patentapplications Nos. 60,162,845, 60/160,182, 60/127,237, 09/535,276 and09/247,887, each of which is assigned to the same assignee as thepresent application, and alternative embodiments are set forth at U.S.Pat. Nos. 5,337,330, 5,818,865 and 5,991,324, all of the above patentsand patent applications being hereby incorporated by reference.

[0041] The solid-state pulser module 14 and high voltage power supply 8supply electrical energy in compressed electrical pulses to thepreionization and main electrodes 3 within the laser chamber 2 toenergize the gas mixture. The preferred pulser module and high voltagepower supply are described at U.S. patent applications Nos. 60/149,392,60/198,058, and 09/390,146, and U.S. patent application of Osmanow, etal., serial number not yet assigned, filed May 15, 2000, for “ElectricalExcitation Circuit for Pulsed Laser”, and U.S. Pat. Nos. 6,005,880 and6,020,723, each of which is assigned to the same assignee as the presentapplication and which is hereby incorporated by reference into thepresent application. Other alternative pulser modules are described atU.S. Pat. Nos. 5,982,800, 5,982,795, 5,940,421, 5,914,974, 5,949,806,5,936,988, 6,028,872 and 5,729,562, each of which is hereby incorporatedby reference. A conventional pulser module may generate electricalpulses in excess of 3 Joules of electrical power (see the '988 patent,mentioned above).

[0042] The laser resonator which surrounds the laser chamber 2containing the laser gas mixture includes optics module 10 includingline-narrowing optics for a line narrowed excimer or molecular fluorinelaser, which may be replaced by a high reflectivity mirror or the likein a laser system wherein either line-narrowing is not desired, or ifline narrowing is performed at the front optics module 12, or anspectral filter external to the resonator is used for narrowing thelinewidth of the output beam. Several variations of line-narrowingoptics are set forth in detail below.

[0043] The laser chamber 2 is sealed by windows transparent to thewavelengths of the emitted laser radiation 14. The windows may beBrewster windows or may be aligned at another angle to the optical pathof the resonating beam. The beam path between the laser chamber and eachof the optics modules 10 and 12 is sealed by enclosures 17 and 19, andthe interiors of the enclosures is substantially free of water vapor,oxygen, hydrocarbons, fluorocarbons and the like which otherwisestrongly absorb VUV laser radiation.

[0044] After a portion of the output beam 20 passes the outcoupler ofthe optics module 12, that output portion impinges upon beam splittermodule 21 which includes optics for deflecting a portion of the beam tothe diagnostic module 18, or otherwise allowing a small portion of theoutcoupled beam to reach the diagnostic module 18, while a main beamportion 20 is allowed to continue as the output beam 20 of the lasersystem. Preferred optics include a beamsplitter or otherwise partiallyreflecting surface optic. The optics may also include a mirror or beamsplitter as a second reflecting optic. More than one beam splitterand/or HR mirror(s), and/or dichroic mirror(s) may be used to directportions of the beam to components of the diagnostic module 18. Aholographic beam sampler, transmission grating, partially transmissivereflection diffraction grating, grism, prism or other refractive,dispersive and/or transmissive optic or optics may also be used toseparate a small beam portion 22 from the main beam 20 for detection atthe diagnostic module 18, while allowing most of the main beam 20 toreach an application process directly or via an imaging system orotherwise. The output beam 20 may be transmitted at the beam splittermodule while a reflected beam portion 22 is directed at the diagnosticmodule 18, or the main beam 20 may be reflected, while a small portion22 is transmitted to the diagnostic module 18. The portion of theoutcoupled beam which continues past the beam splitter module 21 is theoutput beam 20 of the laser, which propagates toward an industrial orexperimental application such as an imaging system and workpiece forphotolithographic applications.

[0045] An enclosure 23 seals the beam path of the beams 22 and 20 suchas to keep the beam paths free of photoabsorbing species. Smallerenclosures 17 and 19 seal the beam path between the chamber 2 and theoptics modules 10 and 12. The preferred enclosure 23 and beam splittingmodule 21 are described in detail in the 09/343,333 and 60/140,530applications, incorporated by reference above, and in U.S. patentapplication Ser. No. 09/131,580, which is assigned to the same assigneeand U.S. Pat. Nos. 5,559,584, 5,221,823, 5,763,855, 5,811,753 and4,616,908, all of which are hereby incorporated by reference. Forexample, the beam splitting module 21 preferably also includes opticsfor filtering visible red light from the beam 22 so that substantiallyonly VUV light is received at a detector of the diagnostic module 18.Filtering optics may also be included for filtering red light from theoutput beam 20. Also, an inert gas purge is preferably flowing throughthe enclosure 23.

[0046] The diagnostic module 18 preferably includes at least one energydetector. This detector measures the total energy of the beam portionthat corresponds directly to the energy of the output beam 20. Anoptical configuration such as an optical attenuator, e.g., a plate or acoating, or other optics may be formed on or near the detector or beamsplitter module 21 to control the intensity, spectral distributionand/or other parameters of the radiation impinging upon the detector(see U.S. patent applications Ser. Nos. 09/172,805, 60/172,749,60/166,952 and 60/178,620, each of which is assigned to the sameassignee as the present application and is hereby incorporated byreference).

[0047] One other component of the diagnostic module 18 is preferably awavelength and/or bandwidth detection component such as a monitor etalonor grating spectrometer (see U.S. patent applications Ser. Nos.09/416,344, 60/186,003, 60/158,808, and 60/186,096, and Lokai, et al.,serial number not yet assigned, “Absolute Wavelength Calibration ofLithography Laser Using Multiple Element or Tandem See Through HollowCathode Lamp”, filed May 10, 2000, each of which is assigned to the sameassignee as the present application, and U.S. Pat. Nos. 4,905,243,5,978,391, 5,450,207, 4,926,428, 5,748,346, 5,025,445, and 5,978,394,all of the above wavelength and/or bandwidth detection and monitoringcomponents being hereby incorporated by reference.

[0048] Other components of the diagnostic module may include a pulseshape detector or ASE detector, such as are described at U.S. patentapplications Ser. Nos. 09/484,818 and 09/418,052, respectively, each ofwhich is assigned to the same assignee as the present application and ishereby incorporated by reference, such as for gas control and/or outputbeam energy stabilization. There may be a beam alignment monitor, e.g.,such as is described at U.S. Pat. No. 6,014,206 which is herebyincorporated by reference.

[0049] The processor or control computer 16 receives and processesvalues of some of the pulse shape, energy, amplified spontaneousemission (ASE), energy stability, energy overshoot for burst modeoperation, wavelength, spectral purity and/or bandwidth, among otherinput or output parameters of the laser system and output beam. Theprocessor 16 also controls the line narrowing module to tune thewavelength and/or bandwidth or spectral purity, and controls the powersupply and pulser module 4 and 8 to control preferably the movingaverage pulse power or energy, such that the energy dose at points onthe workpiece is stabilized around a desired value. In addition, thecomputer 16 controls the gas handling module 6 which includes gas supplyvalves connected to various gas sources.

[0050] The laser gas mixture is initially filled into the laser chamber2 during new fills. The gas composition for a very stable excimer laserin accord with the preferred embodiment uses helium or neon or a mixtureof helium and neon as buffer gas, depending on the laser. Preferred gascomposition are described at U.S. Pat. Nos. 4,393,405 and 4,977,573 andU.S. patent applications Ser. Nos. 09/317,526, 09/513,025, 60/124,785,09/418,052, 60/159,525 and 60/160,126, each of which is assigned to thesame assignee and is hereby incorporated by reference into the presentapplication. The concentration of the fluorine in the gas mixture mayrange from 0.003% to 1.00%, and is preferably around 0.1%. An additionalgas additive, such as a rare gas, may be added for increased energystability and/or as an attenuator as described in the '025 application,mentioned above. Specifically, for the F2-laser, an addition of Xenonand/or Argon may be used. The concentration of xenon or argon in themixture may range from 0.0001% to 0.1%. For an ArF-laser, an addition ofxenon or krypton may be used also having a concentration between 0.0001%to 0.1%.

[0051] Halogen and rare gas injections, total pressure adjustments andgas replacement procedures are performed using the gas handling module 6preferably including a vacuum pump, a valve network and one or more gascompartments. The gas handling module 6 receives gas via gas linesconnected to gas containers, tanks, canisters and/or bottles. Preferredgas handling and/or replenishment procedures of the preferredembodiment, other than as specifically described herein, are describedat U.S. Pat. Nos. 4,977,573 and 5,396,514 and U.S. patent applicationsNos. 60/124,785, 09/418,052, 09/379,034, 60/171,717, and 60/159,525,each of which is assigned to the same assignee as the presentapplication, and U.S. Pat. Nos. 5,978,406, 6,014,398 and 6,028,880, allof which are hereby incorporated by reference. A Xe gas supply may beincluded either internal or external to the laser system according tothe '025 application, mentioned above.

[0052] A general description of the line-narrowing features of theseveral embodiments of the present is first provided here, followed by adetailed discussion referring FIGS. 2a-6 b. Exemplary line-narrowingoptics are contained in the optics module 10 include a beam expander, anoptional etalon and a diffraction grating, which produces a relativelyhigh degree of dispersion, for a narrow band laser such as is used witha refractive or catadioptric optical lithography imaging system. Asmentioned above, the front optics module may include line-narrowingoptics as well (see the 60/166,277, 60/173,993 and 60/166,967applications, each being assigned to the same assignee and herebyincorporated by reference). For a semi-narrow band laser such as is usedwith an all-reflective imaging system, and which is not the subject ofthe present invention, the grating is replaced with a highly reflectivemirror, and a lower degree of dispersion may be produced by a dispersiveprism. A semi-narrow band laser would typically have an output beamlinewidth in excess of 1 pm and may be as high as 100 pm in some lasersystems, depending on the characteristic free-running bandwidth of thelaser.

[0053] The beam expander of the above exemplary line-narrowing optics ofthe optics module 10 preferably includes one or more prisms. The beamexpander may include other beam expanding optics such as a lens assemblyor a converging/diverging lens pair. The grating or highly reflectivemirror is preferably rotatable so that the wavelengths reflected intothe acceptance angle of the resonator can be selected or tuned.Alternatively, the grating, or other optic or optics, or the entireline-narrowing module may be pressure tuned, such as it set forth in the60/178,445 and 09/317,527 applications, each of which is assigned to thesame assignee and is hereby incorporated by reference. The grating maybe used both for dispersing the beam for achieving narrow bandwidths andalso preferably for retroreflecting the beam back toward the laser tube.Alternatively, a highly reflective mirror is positioned after thegrating which receives a reflection from the grating and reflects thebeam back toward the grating to doubly disperse the beam, or the gratingmay be a transmission grating. One or more dispersive prisms may also beused, and more than one etalon may be used.

[0054] Depending on the type and extent of line-narrowing and/orselection and tuning that is desired, and the particular laser that theline-narrowing optics are to be installed into, there are manyalternative optical configurations that may be used. For this purpose,those shown in U.S. Pat. Nos. 4,399,540, 4,905,243, 5,226,050,5,559,816, 5,659,419, 5,663,973, 5,761,236, and 5,946,337, and U.S.patent applications Ser. Nos. 09/317,695, 09/130,277, 09/244,554,09/317,527, 09/073,070, 60/124,241, 60/140,532, 60/147,219 and60/140,531, 60/147,219, 60/170,342, 60/172,749, 60/178,620, 60/173,993,60/166,277, 60/166,967, 60/167,835, 60/170,919, 60/186,096, each ofwhich is assigned to the same assignee as the present application, andU.S. Pat. Nos. 5,095,492, 5,684,822, 5,835,520, 5,852,627, 5,856,991,5,898,725, 5,901,163, 5,917,849, 5,970,082, 5,404,366, 4,975,919,5,142,543, 5,596,596, 5,802,094, 4,856,018, 5,970,082, 5,978,409,5,999,318, 5,150,370 and 4,829,536, and German patent DE 298 22 090.3,are each hereby incorporated by reference into the present application.

[0055] Optics module 12 preferably includes means for outcoupling thebeam 20, such as a partially reflective resonator reflector. The beam 20may be otherwise outcoupled such as by an intraresonator beam splitteror partially reflecting surface of another optical element, and theoptics module 12 would in this case include a highly reflective mirror.The optics control module 14 controls the optics modules 10 and 12 suchas by receiving and interpreting signals from the processor 16, andinitiating realignment or reconfiguration procedures (see the '241,'695, 277, 554, and 527 applications mentioned above).

[0056] A detailed discussion of the line-narrowing configurations of anoscillator element of the laser system according to the preferredembodiment is now set forth with reference to FIGS. 2a-2 f. Severalembodiments of an oscillator of the laser system using line-narrowingtechniques for the molecular fluorine laser, are shown in FIGS. 2a-2 fto meet or substantially meet the first object of the invention.

[0057]FIG. 2a schematically shows an oscillator of a laser systemaccording to a first embodiment including a discharge chamber 2preferably containing molecular fluorine and a buffer gas of neon,helium or a combination thereof (see the 09/317,526 application), andhaving a pair of main discharge electrodes 3 (not shown) and apreionization arrangement (also not shown) therein. The system shown inFIG. 2a also includes a prism beam expander 30 and a diffraction grating32 arranged in a Littrow configuration. The beam expander 30 may includeone or more prisms and preferably includes several prisms. The beamexpander serves to reduce divergence of the beam incident onto thegrating, thus improving wavelength resolution of the wavelengthselector. The grating is preferably a high blaze angle echelle grating(see the 60/170,342 application incorporated by reference above).

[0058] The system shown includes a pair of apertures 34 in the resonatorwhich reject stray light and reduce broadband background, and can alsoserve to reduce the linewidth of the beam by lowering the acceptanceangle of the resonator. Alternatively, one aperture 34 on either side ofthe chamber 2 may be included, or no apertures 34 may be included.Exemplary apertures 34 are set forth at U.S. Pat. No. 5,161,238, whichis assigned to the same assignee and is hereby incorporated by reference(see also the 09/130,277 application incorporated by reference above).

[0059] The system of FIG. 2a also includes a partially reflecting outputcoupling mirror 36. The outcoupling mirror 36 may be replaced with ahighly reflective mirror, and the beam may be otherwise output coupledsuch as by using a polarization reflector or other optical surfacewithin the resonator such as a surface of a prism, window orbeam-splitter (see, e.g., U.S. Pat. No. 5,150,370, incorporated byreference above).

[0060] The system shown at FIG. 2b includes the chamber 2, the apertures34, the partially reflecting output coupling mirror 36 and beam expander30 described above with respect to FIG. 2a. The system of FIG. 2b alsoincludes a diffraction grating 38 and a highly reflective mirror 40. Thegrating 38 preferably differs from the grating 32 of FIG. 2a either inits orientation with respect to the beam, or its configuration such asits blaze angle, etc., or both. The laser beam is incident onto thegrating 38 at an angle closer to 90 ^(E) than for the grating 32. Theincidence angle is, in fact, preferably very close to 90 ^(E). This isarrangement is referred to here as the Littman configuration. TheLittman configuration increases the wavelength dispersion of the grating38. After passing through or reflecting from the diffraction grating 38,the diffracted beam is reflected by the highly reflective mirror 40. Thetuning of the wavelength is preferably achieved by tilting the highlyreflective mirror 40. As mentioned above with respect to the exemplaryarrangement, tuning may be achieved otherwise by rotating another opticor by pressure tuning one or more optics, or otherwise as may beunderstood by one skilled in the art.

[0061]FIG. 2c schematically shows another embodiment of an oscillatorhaving a laser chamber 2, apertures 34, outcoupler 36, beam expander 30and Littrow diffraction grating 32, preferably as described above. Inaddition, the system of FIG. 2c includes one or more etalons 42, e.g.,two etalons are shown, which provide high-resolution line narrowing,while the grating 32 serves to select a single interference order of theetalons 42. The etalon or etalons 42 may be placed in various positionsin the resonator, i.e., other than as shown. For example, a prism orprisms of the beam expander 30 may be positioned between an etalon oretalons 42 and the grating. An etalon 42 may be used as an outputcoupler, as will be described in more detail below with reference toFIGS. 2e-2 f. The arrangement of FIG. 2c (as well as FIG. 2d below)including an etalon or etalons 42 may be varied as described at any ofU.S. patent applications Nos. 60/162,735, 60/178,445, or 60/158,808,each of which is assigned to the same assignee and is herebyincorporated by reference.

[0062]FIG. 2d shows another embodiment of the laser system having one ormore etalons 43, e.g., two etalons 43 are shown. The system of FIG. 2dis the same as that of FIG. 2c except that the grating 32 is replacedwith a highly reflective mirror, and the etalons 43 are differentlyconfigured owing to the omission of the grating 32 which is notavailable, as in the system of FIG. 2c, to select a single interferenceorder of the etalons 43. The free spectral ranges of etalons 43 areinstead adjusted in such a way that one of the etalons 43, preferablythe first etalon 43 after the beam expander 30, selects a single orderof the other etalon 43, e.g., the second etalon 43. The second etalon 43of the preferred arrangement is, therefore, allowed to have a smallerfree spectral range and higher wavelength resolution. Some furtheralternative variations of the etalons 43 of the system of FIG. 2d may beused as set forth in U.S. Pat. No. 4,856,018, which is herebyincorporated by reference.

[0063]FIGS. 2e and 2 f schematically show embodiments similar to thearrangements described above with reference to FIGS. 2a and 2 b,respectively, which differ in that the partially reflecting outcouplermirror 36 is replaced with a reflective etalon outcoupler 46. The etalonoutcoupler 46 is used in combination with the grating 32 or 38 and beamexpander 30 of FIGS. 2e and 2 f, respectively, wherein the grating 32 or38 selects a single interference order of the etalon outcoupler 46.Alternatively, one or more dispersive prisms or another etalon may beused in combination with the etalon outcoupler 46 for selecting a singleinterference order of the etalon 46. The grating 32 or 38 restrictswavelength range to a single interference order of the outcoupler etalon46. Variations of the systems of FIGS. 2e and 2 f that may be used incombination with the systems set forth at FIGS. 2e and/or 2 f are setforth at the 09/317,527 and 60/166,277 applications, incorporated byreference above, and U.S. Pat. Nos. 6,028,879, 3,609,586, 3,471,800,3,546,622, 5,901,163, 5,856,991, 5,440,574, and 5,479,431, and H.Lengfellner, Generation of tunable pulsed microwave radiation bynonlinear interaction of Nd:YAG laser radiation in GaP crystals, OpticsLetters, Vol. 12, No. 3 (March 1987), S. Marcus, Cavity dumping andcoupling modulation of an etalon-coupled CO₂ laser, J. Appl. Phys., Vol.53, No. 9 (September 1982), and The physics and technology of laserresonators, eds. D. R. Hall and P. E. Jackson, at p. 244, each of whichis hereby incorporated by reference.

[0064] In all of the above embodiments shown and described withreference to FIGS. 2a-2 f, the material used for the prisms of the beamexpanders 30, etalons 42, 43, 46 and laser windows is preferably onethat is highly transparent at wavelengths below 200 nm, such as at the157 nm output emission wavelength of the molecular fluorine laser. Thematerials are also capable of withstanding long-term exposure toultraviolet light with minimal degradation effects. Examples of suchmaterials are CaF₂, MgF₂, BaF, BaF₂, LiF, LiF₂, and SrF₂. Also, in allof the above embodiments of FIGS. 2a-2 f, many optical surfaces,particularly those of the prisms, preferably have an anti-reflectivecoating on one or more optical surfaces, in order to minimize reflectionlosses and prolong their lifetime.

[0065] Also, as mentioned in the general description above, the gascomposition for the F₂ laser in the above configurations uses eitherhelium, neon, or a mixture of helium and neon as a buffer gas. Theconcentration of fluorine in the buffer gas preferably ranges from0.003% to around 1.0%, and is preferably around 0.1%. The addition of atrace amount of xenon, and/or argon, and /or oxygen, and/or kryptonand/or other gases may be used for increasing the energy stability,burst control, or output energy of the laser beam. The concentration ofxenon, argon, oxygen, or krypton in the mixture may range from 0.0001%to 0.1%. Some alternative gas configurations including trace gasadditives are set forth at U.S. patent applications Nos. 09/513,025 and09/317,526, each of which is assigned to the same assignee and is herebyincorporated by reference.

[0066] All of the oscillator configurations shown above at FIGS. 2a-2 fmay be advantageously used to produce a VUV beam 20 having a wavelengthof around 157 nm and a linewidth of around 1 pm or less. Some of thoseconfigurations having an output linewidth of less than 1 pm already meetthe above first object of the invention with respect to the linewidth.Those oscillators may be used with other elements, such as an amplifier,as set forth below at FIGS. 3a-6 b to meet the second object of theinvention, i.e., to achieve sufficient output power for substantialthroughput at a 157 nm lithography fab. Other oscillators producinglinewidths above 1 pm may be advantageously used in combination withother line-narrowing elements such as a spectral filter, as set forthbelow at FIGS. 3a-4 b, to meet that first object, and with an amplifieras set forth in the embodiments of FIGS. 3a-4 b to meet the secondobject.

[0067]FIG. 3a schematically illustrates, in block form, a laser systemin accord with a preferred embodiment of the present invention, whereina narrower linewidth is desired than is output by the oscillator 48, andhigher power is desired than is output by the oscillator 48. To reducethe linewidth, the output beam 20 of the oscillator 48 is directedthrough a spectral filter 50. To increase the output power, the beam 20is directed through an amplifier 52.

[0068] The system of FIG. 3a includes a line-narrowed oscillator 48, aspectral filter 50 and an amplifier 52. Various preferred configurationsof the spectral filter 50 are described below with reference to FIGS.3b-3 d. The oscillator 48 of FIG. 3a is an electrical dischargemolecular fluorine laser producing a spectral linewidth of approximately1 pm, and is preferably one of the configurations described above withrespect to FIGS. 2a-2 f, or a variation thereof as described above, oras may be understood as being advantageous to one skilled in the art,such as may be found in one or more of the reference incorporated byreference above. The oscillator 48 is followed by the spectral filter50, which transmits light in a narrower spectral range, i.e., less thanthe linewidth of the output beam 20 from the oscillator or less thanaround 1 pm. Lastly, the transmitted beam is amplified in an amplifier52 based on a separate discharge chamber to yield an output beam 54 thatmeets both the first and second objects of the invention. Preferably,the oscillator and amplifier discharges are synchronized using a delaycircuit and advantageous solid-state pulser circuit such as is describedat U.S. patent application No. 60/204,095 and at U.S. Pat. No.6,005,880, each of which is assigned to the same assignee and is herebyincorporated by reference.

[0069] The spectral filter 50 is preferably includes one of thearrangements shown in FIGS. 3b-3 d. Variations may be understood asadvantageous to one skilled in the art using any of a large number ofcombinations of prisms, gratings, grisms, holographic beam samplers,etalons, lenses, apertures, beam expanders, collimating optics, etc.,for narrowing the linewidth of the input beam 20, preferably withoutconsuming a substantial fraction of the energy of the input beam 20.

[0070]FIG. 3b illustrates a first spectral filter 50 embodimentincluding a beam expander followed by one or more etalons 58 to yield anoutput beam having a linewidth substantially below the linewidth, e.g.,around 1 pm, of the input beam 20 to meet the first object of theinvention. Each etalon 58 includes two partially reflecting surfaces ofreflectivity R, separated by a preferably gas-filled gap of thickness D.The transmission spectrum of the etalon T(λ) is described by a periodicfunction of the wavelength λ:

T(λ)=(1+(4F ²/π²)sin(2πnD cos(Θ)/λ))⁻  (1)

[0071] where n is the refractive index of the material, preferably aninert gas, filling the etalon 58, Θ is the tilt angle of the etalon 58with respect to the beam, and F is the finesse of the etalon 58 which isdefined as:

F=πR ^(½)/(1−R)  (2)

[0072] The reflectivity R and spacing of the etalon D can be selected insuch a way that only a single transmission maximum overlaps with theemission spectrum of the broader-band oscillator 48. For instance, ifthe finesse of the etalon 58 is selected to be 10, then the spectralwidth of the transmission maximum is roughly {fraction (1/10)} of thefree spectral range (FSR) of the etalon 58. Therefore, selecting a freespectral range of 1 pm will produce a transmitted beam with spectrallinewidth of 0.1 pm, without sidebands since the linewidth of theoscillator (48) output (approximately 1 pm) is significantly less thantwo times the FSR.

[0073] Using multiple etalons 58 allows a higher contrast ratio, whichis defined as a ratio of the maximum transmission to the transmission ofthe wavelength halfway between the maxima. This contrast ratio for asingle etalon is approximately equal to (1+4F²/π²). Higher finessevalues lead to higher contrast. For several etalons 58, the totalcontrast ratio will be (1+4F²/π²)^(n) where n is the number of etalons58 used. Additionally, the spectral width of the transmission maximawill be reduced with increased number of etalons 58 used. Disadvantagesof using several etalons 58 include high cost and complexity of theapparatus and increased optical losses.

[0074] The beam expander 56 shown at FIG. 3b serves to reduce thedivergence of the beam incident onto the etalons 58. From the formula(1), it follows that a change in the beam incidence angle ¹ causes ashift of the wavelength at which maximum transmission occurs. Assumingan FSR of 1 pm, the etalon spacing is D=1.2 cm. If the transmissioninterference spectrum of the etalon 58 is at its maximum at normalincidence (Θ=0), then the angle Θ, at which the transmission spectrumreaches maximum again is Θ˜(λ/nD)^({fraction (1/12)})=3.6 mrad.Therefore, it is preferred that the spectral filter 50 shown at FIG. 3bbe configured such that the divergence of the beam is below Θ, andpreferably by a factor comparable to the finesse F of the etalon 58.Since the divergence of a typical molecular fluorine laser is severalmillirads, the advantage of using the beam expander 56 to reduce thisdivergence from typically above Θ as it is output from the oscillator 48to below Θ, is may be understood. It is also preferred to use one ormore apertures 34 in the oscillator 48 to reduce its output divergence(see the 09/130,277 application, mentioned above).

[0075] The gaps between the plates of the etalons 58 are preferablyfilled with an inert gas. Tuning of the transmitted wavelength can beaccomplished by changing the pressure of the gas as described in the09/317,527 application, mentioned above. In addition to pressure tuningand rotation tuning of the etalon's output transmission spectrum, theetalons 58 may be piezoelectrically tuned such as to geometrically alterthe gap spacing.

[0076]FIG. 3c schematically illustrates a second embodiment of thespectral filter 50 of FIG. 3a generally utilizing a diffraction grating60. Although there are other ways to configure the spectral filter 50according to the second embodiment using a grating 60, an example isshown at FIG. 3c and described here. The spectral filter 50 shown atFIG. 3c is a Czerny-Turner type spectrometer, modified to achieve highresolution. The input beam 20 in focused by a lens 61 a through an inputslit 62 a after which the beam is incident on a collimating mirror 64.After reflection from the mirror 64, the beam is incident on a beamexpander 66 and then onto the grating 60. The beam is dispersed andreflected from the grating 60, after which the beam retraverses the beamexpander 66, and is reflected from the collimating mirror 64 through anoutput slit 62 b at or near the focal point of a lens 62 b. The outputbeam 59 then has a linewidth substantially less than the linewidth,e.g., around 1 pm, of the input beam 20, or substantially less than 1 pmto meet the first object of the invention.

[0077] The diffraction grating 60 is preferably a high blaze echellegrating 60. The wavelength dispersion of this preferred grating 60 isdescribed by the following formula:

dλ/dΘ=(2/λ) tan Θ  (3)

[0078] where Θ is the incidence angle. The spectral width Δλ of thetransmitted beam is determined by the dispersion dλ/dΘ of the grating60, the magnification factor M of the prism expander 66, the focallength L of the collimating mirror 64 and the width d of the slits 62 a,62 b of the spectrometer:

Δλ=d(L M dλ/dΘ)⁻¹  (4)

[0079] For example, using an echelle grating 60 wherein the incidenceangle Θ is 78.6°, L=2 m and M=8, the slit width d which would achieve0.1 pm resolution for the spectral filter 50 of FIG. 3c is around d=0.1mm. It is preferred, therefore, to reduce the divergence of theoscillator 48 in order to increase the transmission of the beam 20through the input slit 61 a. This can be advantageously achieved byusing apertures inside the resonator of the oscillator 48 (see again the09/130,277 application, mentioned above).

[0080] The third example of a spectral filter 50 that may be used inillustrated at FIG. 3d. The spectral filter 50 of FIG. 3d differs fromthat shown at FIG. 3c in that a collimating lens 68 is used in theembodiment of FIG. 3d, rather than a collimating mirror 64, as is usedin the embodiment of FIG. 3c. An advantage of the embodiment of FIG. 3dis its simplicity and the absence of astigmatism introduced by themirror 64 of FIG. 3c at non-zero incidence angle.

[0081] It is useful to reiterate here that synchronization of theelectrical discharge pulses in the chambers 2 of the oscillator 48 andamplifier 52 is preferred in order to ensure that the line-narrowedoptical pulse from the oscillator 48 arrives at the chamber 2 of theamplifier 52 at the instance when the gain of the amplifier 52 is at ornear its maximum. Additionally, this preferred synchronization timingshould be reproducible from pulse to pulse to provide high energystability of the output pulses. The preferred embodiment electroniccircuitry allowing this precise timing control is described at U.S. Pat.No. 6,005,880 and U.S. patent application No. 60/204,095, as mentionedabove.

[0082]FIG. 4a shows the use of a single discharge chamber 70 thatprovides the gain medium for both an oscillator and an amplifier. Thesetup of FIG. 4a includes the discharge chamber 70 within a resonatorincluding a highly reflective mirror 72 and a partially reflectingoutcoupling mirror 74. A pair of apertures 34 are also included, asdescribed above, to match the divergence of the resonator of thisoscillator 48. A small portion of the cross-section of the dischargevolume is used to produce an un-narrowed beam 76 with this oscillatorconfiguration. It is also possible to include one or more line-narrowingcomponents with this oscillator configuration, or to otherwise modifythe oscillator according to the description set forth above with respectto FIGS. 2a-2 f.

[0083] Similar to the embodiment shown and described with respect toFIG. 3a, this un-narrowed output is then directed through a spectralfilter 50, which is preferably one of the embodiments described in FIGS.3b-3 d. Given the significant time (e.g., several nanoseconds) that ittakes for the beam to traverse the spectral filter 50, it is preferredto adjust the arrival time of the filtered pulse to a second maximum ofthe discharge current. To achieve this temporal adjustment, an opticaldelay line is preferably inserted after the spectral filter 50. Thedelay line may be one of those described at U.S. patent application No.60/130,392, which is assigned to the same assignee and is herebyincorporated by reference.

[0084]FIGS. 4b (i)-(iii) illustrate the electrical current through thedischarge gap, the intensity of the un-narrowed beam 76 and the output59 of the oscillator-amplifier system, each as a function of time. Thecurrent exhibits several cycles of oscillations, as shown in FIG. 4b(i). The optical pulse shown at FIG. 4b (ii) evolves towards the end ofthe first maximum (a) of current. The second maximum of electricalcurrent is separated from the first one by approximately 20 nanoseconds,thus providing sufficient time for the beam 76 to traverse the spectralfilter 50 and additional optical delay line 78. This discussion withrespect to the timing of the successive maxima in the electricaldischarge current reveals how the additional optical delay line 78 maybe advantageously used to precisely tune the arrival time of the pulseat the chamber 70 (amplifier). The line-narrowed beam from the spectralfilter 50, whose temporal pulse shape is shown at FIG. 4b (iii), thusoverlaps the second maximum b of the electrical current shown at FIG. 4b(i) of the amplifier and is amplified, and thus a line-narrowed beam 59,i.e., substantially less than 1 pm, is output with sufficient power tomeet the first and second objects of the invention.

[0085]FIG. 5a shows the use of a line-narrowed oscillator followed by apower amplifier made in a separate discharge chamber. Any of theembodiments shown and described above including those discussed withrespect to the exemplary embodiments, the patents and publicationsincorporated by reference,and the embodiments described with respect toFIGS. 2a-2 f can be used to narrow the bandwidth of the oscillator.Examples of the preferred line-narrowed oscillator 48 are set forth atFIGS. 5b-5 f.

[0086] The line-narrowed oscillator 48 schematically shown at FIG. 5(b)uses a prism beam expander 30 and grating 32, preferably as described inone or the U.S. Pat. No. 5,559,816 298 22 090.3 DE, U.S. Pat. No.4,985,898 U.S. Pat. No. 5,150,370 and U.S. Pat. No. 5,852,627 patents,each being incorporated by reference above. Alternatively, the Littmanconfiguration may be used (see discussion above with respect to FIG.2b). As discussed above with respect to the embodiments of FIGS. 2a-4 a,the additional apertures 34 in the resonator reduce divergence of thebeam and, therefore, advantageously increase the resolution of thewavelength selector (again, see the 09/130,277 application for details).

[0087] The embodiment shown in FIG. 5c utilizes multiple etalons 43 aswavelength selective elements (see FIG. 2d). The prism beam expander 30in combination with the apertures 34 helps to reduce the divergence ofthe beam in the etalons 43 thus improving resolution of the wavelengthselector. Additionally, this reduces the intensity of the beam at aparticular area of the surfaces of the etalons 43, thus extending theirlifetime.

[0088]FIGS. 5d-5 e show alternative arrangements that each include an RFor microwave excited waveguide laser as an oscillator. The arrangementof FIG. 5d preferably includes a pair of RF-electrodes 80 and awaveguide 82 preferably including a ceramic capillary filled with alaser active gas mixture. Any of the resonator configurations shown inFIGS. 2a-5 c may be used in this embodiment, wherein the dischargechamber 2 is replaced with the RF-excited waveguide arrangement shown inFIG. 5d. Features of the waveguide laser that may be used in thearrangement of FIGS. 5d-5 e may be found at C. P. Christenson, CompactSelf-Contained ArF Laser, Performing Organization Report Number AFOSR IR95-0370; T. Ishihara and S. C. Lin, Theoretical Modeling ofMicrowave-Pumped High-Pressure Gas Lasers, Appl. Phys. B 48, 315-326(1989); and Ohmi, Tadahiro and Tanaka, Nobuyoshi, Excimer LaserOscillation Apparatus and Method, Excimer Laser Exposure Apparatus, andLaser Tube, European Patent Application EP 0 820 132 A2, each of whichis hereby incorporated by reference. RF-excited lasers are commonlyoperated with a carbon dioxide gas medium, e.g., as discussed in KurtBondelie “Sealed carbon dioxide lasers achieve new power levels”, LaserFocus World, August 1996, pages 95-100, which is hereby incorporated byreference.

[0089] The specific arrangement shown in FIG. 5d includes a prism beamexpander 30 and a grating 32 in Littrow configuration. A Littmanconfiguration may be used here (see FIGS. 2b and 2 f) including thegrating 38 and HR mirror 40. A pair of apertures 34 are again included,particularly for matching the divergence of the resonator. A partiallyreflecting mirror 36 outcouples the beam 20. An etalon outcoupler 46 maybe used instead of the mirror 36 (see FIGS. 2e-2 f)

[0090] The arrangement schematically shown at FIG. 5e is the same asthat of FIG. 5d, except that the grating is replaced with a one or moreetalons 43 and an HR mirror 44. A grating 32 or 38 may be used alongwith the etalons 43, and an etalon outcoupler 46 may be used instead ofthe partially reflecting mirror 36.

[0091] An advantage of this RF-excited waveguide type of laser is itslong pulse, which allows more efficient line narrowing, since thelinewidth is approximately inversely proportional to the number of roundtrips of the beam in the resonator. Additionally, the RF-excitedwaveguide laser has a small discharge width (on the order of 0.5 mm)which allows high angular resolution of the wavelength selector based onthe prisms of the beam expander 30 and the diffraction grating 32. Thisholds for both of the embodiments shown at FIGS. 5d-5 e.

[0092]FIG. 5f schematically shows another source of a narrow linewidthbeam that may be used in accordance with the present invention to serveas the oscillator 48 in the embodiment of FIG. 5a. The arrangement ofFIG. 5f includes a solid state laser 85 with a third harmonic output at355 nm, such as diode pumped, Nd:YAG laser or other such type laser asmay be described, e.g., at U.S. Pat. No. 6,002,697, which is assigned tothe same assignee and is hereby incorporated by reference, or as may beotherwise known to one skilled in the art. The solid state laser 85, inturn, pumps a narrow linewidth tunable laser 86, such as a dye laser oroptical parametric oscillator, emitting, e.g., around 472.9 nm. This472.9 nm radiation is focussed into a gas cell 88 containing a mixtureof halide metal and inert gas, in order to produce a third harmonic beamat 157.6 nm. Such third harmonic generation in gases has been describedat: Kung A. H., Young J. F., Bjorklung G. C., Harris S. E., PhysicalReview Letters, v.29, Page 985 (1972); and Kung A. H., Young J. F.,Harris S. E, Applied Physics Letters, v.22 page 301 (1973), each ofwhich is hereby incorporated by reference.

[0093]FIGS. 6a and 6 b schematically illustrate further embodimentswherein a portion of the discharge volume of a discharge chamber 2 isused as an oscillator with line narrowing, and the same dischargechamber 2 is used as an amplifier 52. The arrangement of FIG. 6a issimilar to that shown at FIG. 4a except that the linewidth of the beam30 is narrowed within the resonator of the oscillator, and no spectralfilter 50 is preferably used. A spectral filter 50 may alternatively beused in addition to the line-narrowing optics of the oscillator of FIG.6a. Again, the line-narrowing arrangement of the oscillator may bemodified as set forth in any of the descriptions above (see particularlyFIGS. 2a-2 f, 5 c and 5 f), or as set forth in any of the patents,patent applications or publications incorporated by reference in thisapplication, or as otherwise understood by one skilled in the art, toproduce a narrow output beam 20 sufficient to meet the first object ofthe invention. The output beam 20 from the oscillator is expanded by anexternal beam expander 90, preferably comprising one or more prisms andalternatively comprising a lens arrangement.

[0094] The expanded beam 92 is then directed through a delay line 78(see the '392 application) to synchronize the pulse with theamplification maxima of the chamber 70, as described above. The opticaldelay line 78 serves to fine tune the arrival time of the optical pulseto the amplifier section, similar to the embodiment shown and describedwith respect to FIGS. 4a-4 b (iii). The expanded beam 20 thenadvantageously fills a substantial portion of the rest of the dischargecross section, and is amplified.

[0095] In the above embodiments, it is preferred to adjust the gasmixture in the discharge chamber 2, 70 of the oscillator, to obtain thelongest possible pulse. Additionally, the waveform of the dischargecurrent can be modified by deliberately introducing an impedancemismatch of the pulse forming circuitry and discharge gap. The impedancemismatch leads to a longer discharge time and thus, to a longer opticalpulse. The lower gain resulting from such modification means lowerefficiency of the oscillator. However, in the embodiments discussedabove, the amount of reduction in the output power of the oscillator isregained at the amplification stage.

[0096] While exemplary drawings and specific embodiments of the presentinvention have been described and illustrated, it is to be understoodthat that the scope of the present invention is not to be limited to theparticular embodiments discussed. Thus, the embodiments shall beregarded as illustrative rather than restrictive, and it should beunderstood that variations may be made in those embodiments by workersskilled in the arts without departing from the scope of the presentinvention as set forth in the claims that follow, and equivalentsthereof.

[0097] In addition, in the method claims that follow, the steps havebeen ordered in selected typographical sequences. However, the sequenceshave been selected and so ordered for typographical convenience and arenot intended to imply any particular order for performing the steps,except for those claims wherein a particular ordering of steps isexpressly set forth or understood by one of ordinary skill in the art asbeing necessary.

What is claimed is:
 1. An excimer or molecular fluorine laser,comprising: an oscillator for generating a pulsed sub-0.6 nm, sub-250 nmlaser beam, including: a laser tube including a discharge chamber filledwith a laser gas mixture at least including molecular fluorine and abuffer gas; a plurality of electrodes in the discharge chamber connectedto a pulsed discharge circuit for energizing the gas mixture; aresonator surrounding the gas mixture for generating a pulsed sub-250 nmlaser beam; and a line-narrowing unit for narrowing the bandwidth ofsaid laser, said line-narrowing unit including a beam expander and agrating and narrowing said bandwidth to less than 0.6 pm, and anamplifier for increasing an energy of the pulsed sub-0.6 pm, sub-250 nmlaser beam, including a laser tube including a discharge chamber filledwith a laser gas mixture at least including molecular fluorine and abuffer gas; a plurality of electrodes in the discharge chamber connectedto a pulsed discharge circuit for energizing the gas mixture at timeswhen pulses of the sub-250 nm laser beam generated by the oscillator arepresent within the discharge chamber; and a resonator surrounding thegas mixture for generating a laser beam.
 2. The laser of claim 1,wherein said bandwidth is less than 0.5 pm.
 3. The laser of claim 1,wherein said bandwidth is less than 0.4 pm.
 4. The laser of claim 1,further comprising a spectral filter between the oscillator and theamplifier for further narrowing the bandwidth of the pulsed sub-0.6 nm,sub-250 nm laser beam output by the oscillator.
 5. The laser of claim 1,wherein the line-narrowing unit further comprises an etalon for furthernarrowing the bandwidth of the pulsed sub-0.6 nm, sub-250 nm laser beamoutput by the oscillator.
 6. The laser of claim 1, further comprising amirror disposed between the beam expander and the grating.
 7. The laserof claim 1, wherein no other optics are disposed between the beamexpander and the grating.
 8. The laser of claim 1, wherein pulses outputby the oscillator have energies below a target energy and wherein theenergies of the pulses are increased to approximately the target energyby the amplifier.